Japanese Knotweed (Polygonum cuspidatum)
Sieb. & Zucc.
Siebold & Zucc.
Fallopia japonica var. japonica
(Houtt.) Ronse Decr.
Miller, J.H., E.B, Chambliss, N.J. Loewenstein. 2010. A Field Guide for the Identification of Invasive Plants in Southern Forests. General Technical Report SRS-119. Asheville, NC. United States Department of Agriculture, Forest Service. 126 p.
Tall perennial, herbaceous shrub 3 to 12 feet (1 to 3.5 m) high, freely branching in dense, often clonal, infestations. Reddish stems, hollow and jointed like bamboo, survive only 1 season while rhizomes up to 65 feet (20 m) long survive decades. Alternate leaves appear in spring on new sprouts, ovate with pointed tips and flat bases. In late summer, sprays of tiny, white flowers emerge along stalks at leaf axils, yielding abundant tiny-winged seeds. Dead plants remain upright or leaning during winter and burn hot to pose a severe fire hazard.
Round, reddish brown to mottled with green, to about 1 inch (2.5 cm) in diameter, resembling bamboo although not woody, smooth with scattered to many tiny dots (scales), often ridged, having hollow internodes and swollen solid nodes with membranous sheaths clinging to the base of the nodes. Profuse red to green, slender branches grow upward and outward, and some drooping to form dense entanglements.
Alternate and broadly ovate to oblong ovate, 4 to 6 inches (10 to 15 cm) long and 3 to 4 inches (8 to 10 cm) wide with distinctly pointed tips and straight wedge bases. Smooth and bright green above with whitish indented veins and dull green beneath with protruding veins. Petioles reddish, 0.5 to 1 inch (1.2 to 2.5 cm) long. Leaves turn bright yellow in fall.
May to September. Terminal and axillary, branched sprays (racemes) 3 to 6 inches (8 to 15 cm) long, covered with tiny 5-petaled (sepaled) white to greenish flowers all having 3 styles and 8 to 10 stamens. Functionally male or female flowers can occur on different plants or within a raceme.
Fruit and seeds
August to November. Many dangling, winged fruit that can contain 1 triangular, shiny nutlet. Viability apparently variable but can be quite high in some stands.
Tolerates a wide range of growing conditions from full sun to shade, to high salinity and drought, while it prefers wet soils in low places or along streams and rivers. Spreads along streams by stem and rhizome fragments and seeds to dominate extensive riparian habitat. Also spreads along highways and roads by similar means through maintenance mowing. A serious threat to native habitats since the dense infestations exclude all other plants and animals.
Resembles the nonnative invasive giant knotweed (P. sachalinense F. Schmidt ex Maxim.) a larger plant with greenish flowers and cordate leaves with tapering points, currently found in KY, VA, TN, NC, and LA. These invasives hybridize in the Northeast (NE).
History and use
Introduced from China, Japan, and Taiwan in the late 1800s as an ornamental. An invasive in many parts of the world.
Found in VA, KY, TN, and NC with scattered occurrences elsewhere except in OK, TX, and FL.
- Do not plant. Remove prior plantings, and control sprouts and seedlings. Bag and dispose of fruit in a dumpster or burn.
- Minimize disturbance within miles of where this plant occurs, and anticipate wider occupation when plants are present before disturbance.
- Repeated cutting and pulling will not control this species unless young.
- Burns hot in dormant season to clear tops with rhizomes remaining.
Recommended control procedures
- Thoroughly wet all leaves with one of the following herbicides in water with a surfactant: Garlon 3A (or aquatic Renovate) as a 2-percent solution (8 ounces per 3-gallon mix); better results may occur from a mix of Garlon 3A as a 1-percent solution (4 ounces per 3-gallon mix) and a glyphosate herbicide (Rodeo for aquatic sites) as a 2-percent solution (8 ounces per 3-gallon mix). Fall applications are most effective while seed production where it occurs can be stopped by earlier treatments.
- On terrestrial sites when safety to nontarget vegetation is not a concern, thoroughly wet all leaves with Arsenal AC* as a 0.25-percent solution (1 ounce per 3-gallon mix), Arsenal PowerLine* as a 0.5-percent solution (2 ounces per 3-gallon mix), Tordon 101* ‡ as a 4-percent solution (1 pint per 3-gallon mix), or Tordon K* ‡ as a 2-percent solution (8 ounces per 3-gallon mix).
- On aquatic sites, thoroughly wet all leaves at flower plume stage with Habitat* as a 1-percent solution (4 ounces per 3-gallon mix) mixed with an aquatic surfactant.
- For stems too tall for foliar sprays, cut large stems and immediately treat the stump tops with one of the following herbicides: a glyphosate herbicide or Garlon 3A as a 25-percent solution (3 quarts per 3-gallon mix). ORTHO Brush-B-Gon, Enforcer Brush Killer, and Vine-X are effective undiluted for treating cut-stumps and available in retail garden stores (safe to surrounding plants). A subsequent foliar application of glyphosate will be required to control new seedlings and resprouts.
* Nontarget plants may be killed or injured by root uptake.
‡ When using Tordon herbicides, rainfall must occur within 6 days after application for needed soil activation. Tordon herbicides are restricted use pesticides.